1. Field of the Invention
This invention relates to a multimedia communication apparatus for performing transmission/reception of image information and voice information via a digital communication network, and more particularly, to a multimedia information communication apparatus which is suitable for storing encoded data.
2. Description of the Related Art
In the case of using a conventional analog telephone circuit, a telephone can transmit only voice, and data can be transmitted only at a low speed.
Recently, however, as a result of progress in communication technology, semiconductor technology and optical technology, digital networks have been provided, and it becomes possible to transmit data having a large capacity at a high speed.
Digital transmission has features in that, for example, the same level of data quality can be maintained without degrading the data quality during data transmission, a particular transmission channel corresponding to the characteristics of the medium of transmitted data is not required, and therefore all media can be dealt with. Hence, it becomes possible to perform transmission between complex media terminals. As a result, multimedia communication apparatuses have appeared, in which a telephone can silmultaneously transmit images as well as voice.
Transmitted voice data may comprise 64 Kbps (.mu.-law, A-law), 64 Kbps/56 Kbps/48 Kbps (SB-ADPCM), 32 Kbps (AD-PCM), 16 Kbps, 8 Kbps, or the like.
Transmitted image data having a large capacity is recuded to data having a small capacity after being subjected to band compression using various methods, such as movement compensation, frame skipping, interframe compensation, intraframe compensation, DCT (discrete cosine transform), vector quantization transform, and the like, so that the image data can be transmitted through a digital network. H. 261 recommended by ITU-T (which succeeds CCITT) is an image encoding method in which data can be transmitted at a transmission speed of 64 Kbps of a basic interface of an ISDN (integrated services digital network). International standardization by the ITU-T, and the like has been intended so that mutual communication can be performed between terminals which deal with voice data and image data, and service provisions, protocol provisions, provisions for the configuration of multimedia multiplexed frames for AV (audiovisual) services, such as video telephones, video conferences, and the like using a digital network, have been announced as H. 320, H. 242, H. 221, and the like recommended by the ITU-T.
In H. 221, the frame configuration, exchange of terminal capabilities, and code allocation of an FAS (frame alignment signal) and a BAS (bit allocation signal) in a communication mode in AV series for 64 Kbits to 1920 Kbits are defined.
In H. 242, protocols for exchange of capabilities between AV terminals using a BAS, the switching of the communication mode, and the like are defined. In H. 320, system aspects for all AV services are defined.
In the above-described recommendations, methods for performing communication of multimedia, comprising images, voice, data, and the like, between terminals according to procedures, such as a sequence of in-channel exchange of terminal capabilities using a BAS, a mode-switching sequence by assigning a communication mode, and the like, after setting of an end-to-end physical connection and in-channel establishment of synchronization using an FAS are provided.
However, methods for changing capabilities of a terminal in accordance with a situation, and methods of selecting a communication mode within the range of exchanged capabilities are not provided.
In multimedia communication, the information transfer speed for voice information is determined by assigning a voice-encoding method, and the transfer speed of data information is determined by assigning the value of the transfer speed when data information is used. The transfer speed for image information is obtained by subtracting the transfer speed of voice information and the transfer speed of data information from the information transfer speed of the entire communication channel which has been set.
A function of performing a recording operation during absence is desired in multimedia communication apparatuses, such as video telephones, and the like, as well as in ordinary telephone sets which transmit only voice.
Response messages (images and/or voice) to be transmitted in an recording operation during absence may be subjected analog recording on a magnetic tape, or the like, and image information and voice information received during absence may be decoded into analog signals and may be recorded on a magnetic tape. However, such an approach results in a large amount of stored information, thereby causing troublesome control of stored information, the necessity of the control of a tape-driving system, and an increase in the size of the entire system. Furthermore, alanog recording causes inconvenience in reutilization of received images, and the quality of images and voice is degraded due to reproduction by decoding.
Accordingly, it is desirable to perform digital recording of received signals, particularly, received image signals, and to store the signals in a recording medium in an encoded state.
According to the above-described recommendations, capabilities of terminals in communication are exchanged when starting the communication. Capabilities to be exchanged between terminals comprise, for example, kinds of information which can be dealt with (images, voice and data), encoding methods, and the transfer rate of each information to be transmitted. The transmission-side terminal transmits information, which belongs to the kind of information selected within the range of capabilities of the reception-side terminal, in permissible encoding method and transfer rate in accordance with the result of the exchange of capabilities between the terminals.
Even during communication, the kind of information to be transmitted, the encoding method, the transfer rate, and the like can be changed within permissible ranges. Accordingly, the comminication partner's terminal may change any of the transmission capabilities in the course of a recording operation during absence. Hence, for example, even if the kind of communicated information is changed, information before and after the change must be stored in a correlated state, and must be appropriately reproduced whenever necessary.
A response message transmitted in a recording operation during absence must be adapted to the capabilities of the communication partner's terminal. That is, the message must be encoded in an encoding method which can be decoded by the communication partner's terminal. If the communication partner's terminal does not have capabilities to receive the response message, the response message cannot be transmitted to the communication partner's terminal (or cannot be reproduced by the communication partner's terminal).